1. Field of the Invention
This invention relates generally to fiber optic connectors and ferrules therefor and more particularly to an improved ferrule which minimizes subsequent end polishing required after the optical fiber has been mechanically secured within the ferrule.
2. Description of Related Art
Fiber optic cables are utilized for carrying various forms of signals in countless industrial installations, in equipment and in apparatus, and in consumer and commercial products of all types. Typically, each end of the optical fiber held within a deformable ferrule which is secured within a connector must be polished in a plane generally orthogonal to the length of the fiber optic cable for proper signal transmission to occur with a mating optical fiber element.
To accomplish the polishing of the end of the optical fiber, as well as to provide support structure to effect the mechanical engagement between fiber optic cable ends, the fiber optic connector such as that shown in FIGS. 7 and 8 of U.S. Pat. No. 5,305,406 invented by Rondeau and incorporated by reference includes a metallic or deformable ferrule or core 166 mechanically attached around the optical fiber 168 extending from the buffer 232. The ferrule 168 is secured within the surrounding connector 162 for additional stabilizing support of the ferrule 168. An additional metallic sleeve 230 is also provided to interconnect the distal portion of the buffer 232 and the proximal end of the ferrule 168 as best seen in FIG. 8 of the '406 patent.
Both the '406 patent and U.S. Pat. No. 6,510,271 invented by applicant herein, disclose apparatus and methodology to reduce the amount of projecting optical fiber which extends beyond the distal tip of the ferrule 168 and to minimize the amount of looseness or side clearance between the optical fiber and the distal end portion of the ferrule 168 after it has been mechanically crimped or swaged around the optical fiber to mechanically secure this relationship. Thereafter, a small amount of the projecting optical fiber and the distal end face surface of the ferrule is typically polished substantially flat and orthogonal to the longitudinal axis of the ferrule so as to maximize the signal transmission between polished end surfaces of adjoining optical fiber connections. However, alternate polished distal tip ends such as radiused or tapered for specialized situations, are also used.
The current prior art is shown in FIGS. 1 to 11. Referring first to FIGS. 1 to 4, a number of metallic deformable ferrules which are used in conjunction with the method disclosed in the '406 patent are there shown generally at numerals 1, 1a, 1b and 1c. This type of ferrule has been utilized for a number of years in standard SMA, ST, SC, and FC connectors as well as in custom fiber optic connectors. These metallic ferrules are normally held at the rear or proximal end portion 9 and 9c to the body of a connector previously described in FIGS. 7 and 8 of the '406 patent. The connector itself forms no portion of the present invention and is being described for reference only.
Each of these ferrules 1, 1a and 1b include an elongated cylindrical body 3, 3a and 3b, respectively, each having a hollow cylindrical interior 10 open at a proximal end thereof to receive an end portion of the buffer or protective sheath around the optical fiber or optical fiber bundle. The exposed optical fiber is inserted through the proximal end 7 of the ferrule body and, with respect to the prior art ferrule embodiment of FIGS. 1 to 3, is passed through the cylindrical cavity 10 and is guided by tapered transitional region 11 into and through a longitudinal optical fiber bore 6 to extend longitudinally beyond the distal end face 2.
With respect to the prior art embodiment 1c of FIG. 4, the optical fiber bore 6c extends longitudinally through almost the entire ferrule body 3c from the tapered transition 11a which also defines the proximal opening 7c of the proximal end portion 9c to the distal end face 2c of the distal end portion 8c. 
After the optical fiber 12 has been inserted through the optical fiber bore 6 as seen in FIG. 5 with the buffer 12a also inserted fully into the cylindrical interior 10, an impact forming or an impact swaging tool 13 having a truncated conical opening 16 defining a conical or tapered surface 14 and a longitudinal cylindrical bore 15 is brought together against the edge 5 of the distal end portion 8a, 8b or 8c of FIGS. 1 to 4, to mechanically deform the reduced diameter distal end portion 4 so as to mechanically crimp and frictionally engage the optical fiber 12 within the deformed bore 6′ in FIG. 6. Note in FIG. 5 that the distal end face 2 is initially flat and orthogonal with respect to the longitudinal axis and bore 6 of the ferrule 50 itself.
In FIG. 6, the tool 13 has been forcibly urged against the outer distal corner 5 of the now deformed distal end portion 4′ so as to cause inward deformation of this region at 17 of the ferrule in compliance against the tapered surface 14. Several deformations occur during this impact swaging or impact forming operation, the first of which is that the outer corner 5 are severely deformed inwardly so as to at least partially collapse the bore 6′ in the region 18. The deformable material of the ferrule at deformed surface 17 causes tightening around the optical fiber 12 in the region 18 to reduce and eliminate any clearance which has been pre-established by the sizing between the diameter of the optical fiber 12 and that of the undeformed bore 6, now 6′ when deformed. Additionally, the previously flat distal end face 2 has now taken a dish or crater configuration 19 with the distal end portion of the optical fiber 12 extending longitudinally therebeyond. Typically, the length of the optical fiber gripping region 18 is in the range of 0.2 mm, creating a frictional resistance to movement of the optical fiber 12 in the range of approximately 400 gms.
The depth of the concaved crater 19 as referenced in FIG. 7 is in the range of 0.1 mm and obviously will increase in proportion to the amount of force exerted by tool 13. The excess projection of the optical fiber 12 must be removed before the end polishing operation is commenced. To do this, typically the optical fiber 12 is cleaved at cutting line 20a in FIG. 7. However, the cleaving operation is typically only able to sever the optical fiber 12 in a range of approximately 0.05 mm at 20a from the edges of the crater 19. Thereafter, an end grinding and polishing operation must reduce the remaining exposed portion of the optical fiber 20a in FIG. 8. Manual cleaving or the use of a cleaving device such as that shown in the '271 patent may be utilized after the optical fiber 12 has been cleaved to establish distal end face 2d. The projecting small portion of optical fiber 12, along with the crater 19 must be ground or sanded and polished down to the bottom of the crater at 21 to create preferably a substantially flat orthogonal surface as best seen in FIG. 11. During the grinding or sanding and polishing operation, a lateral force in the direction of the array of arrows shown in FIG. 10 is imposed along the gripping area 18 between the optical fiber 12 and the deformed bore 6′. As a result, the gripping force in gripping region 18 may be reduced by as much as 50% or more or even totally lost, rendering the ferrule unusable. Moreover, the overall length of this gripping region 18 may be reduced if excess material from the ferrule as well as the optical fiber 12 is removed inadvertently or carelessly during this polishing operation. The variables which affect the overall quality of this gripping force in this prior art arrangement are controlled by the impact of the swaging die process shown in FIG. 6, the reduced diameter of the deformable tip 4, and the angle of the conical surface 14 of the swaging tool 13.